Aneuploids

Aneuploidies are chromosomal abnormalities characterized by the presence of an abnormal number of chromosomes in a cell or organism. Unlike aneuploidies, which remove or add whole chromosomes, this condition involves a gain or loss of one or more entire chromosomes, resulting in a genetic dosage imbalance. Aneuploidies are typically generated during cell division, most often during meiosis, through errors in chromosome segregation such as nondisjunction. They can also arise during mitotic divisions in somatic cells, leading to mosaic aneuploidy. The consequences of aneuploidy vary according to the chromosome involved, the species, and the developmental stage. In humans, the most common disorders are Down syndrome (trisomy 21), Turner syndrome (45,X), Klinefelter syndrome (47,XXY), and Cri‑du‑eau syndrome (deletion of part of chromosome 5). In other species, aneuploidies underlie certain cancers, for example the frequent gains and losses of chromosomes 7, 5, and 17 in colorectal cancer, or the extra copies of chromosome 7 and 13 in chronic myeloid leukemia.

Aneuploidy can affect cellular function by disrupting the stoichiometric balance of protein complexes, co-regulated transcription factors, and metabolic pathways. Cells often activate stress response pathways, including the unfolded protein response, to cope with the altered protein load. In multicellular organisms, whole-chromosome gain or loss can impair organ development, reduce fertility, or increase susceptibility to disease. Very rare cases of whole-genome duplication, a related event, have contributed to speciation in plants and some animal lineages.

Detection methods include karyotyping, fluorescent in situ hybridization (FISH), and more recently, comparative genomic hybridization (CGH) microarrays and next-generation sequencing (NGS)-based copy-number variation analyses. These tools enable precise determination of chromosomal dosage and identification of novel aneuploidies. Early detection of aneuploidy in clinical settings informs prognostic assessment, guides therapeutic decisions, and provides insights into underlying developmental biology.